One type of a light source is the fluorescent tube. In the fluorescent tube a gas discharge emits ultraviolet (UV) light onto a fluorescent material. The light source suffers from serious drawbacks. For instance, there is always a delay after the power has been turned on until the light source radiates at full power. Further, it needs complicated control equipment, which requires space and adds cost. Also, it is unfortunately necessary to use material having negative environmental effects, such as mercury. Furthermore, the choice of fluorescent material is limited to UV-sensitive materials. Most of these fluorescent materials emit light of a spectral shape, which is not optimal for the eye and human comfort. Finally, this kind of light source is often rather temperature sensitive in that the emission intensity is significantly weaker for a long time after switch-on at low temperatures compared to at high temperatures.
Another type of light source is the cathodoluminescent light source. In a cathodoluminescent light source electrons are emitted from a cathode either by heating the cathode, thus thermally emitting the electrons, or by employing a strong electric field in the vicinity of the surface of the cathodes, thus emitting electrons through field emission.
Examples of field emission cathode light sources employing a strong electric field in the vicinity of the surface of a cathode are disclosed in U.S. Pat. Nos. 5,877,588 and 6,008,575.
The main drawback of a thermally emitting cathode is that large amounts of energy are lost in heating the cathode. The main drawback of both field and thermal emission cathodes is that high emission currents cause the cathode to wear out as all electrons producing the light have to be emitted from the cathode. This implies that a high electron current has to be emitted from the surface of the cathode, which complicates the cathode structure and production thereof. Further, the current cathodoluminescent light sources only operate in vacuum, which requires thick walls around the light source.